Sculpture UV & Paint Issue
There is another way to quickly do the shading without the need of retopology and UV unwraping if are not going to animate the model. You can use the texture cordinate node to change the cordination from UV to project or other ways... then using some procedural or seemles textures and mix them together with a balck and white mask as a factor instead of texture painting. Highly recommend to watch the "Materials Made Easy - Non Destructive Shading in Blender" by Zacarist Reinhardt in Blender's YouTube channel from Blender Con 2018. (Sorry having internet problem I couldn't get you a link)